The invention relates to an information processing apparatus having a function which can display an image by a television signal of the NTSC or the like and, more particularly, to an information processing apparatus for making it possible to output and display a synthetic image in which an influence by a jitter of a television signal inputted from the outside is suppressed to a minimum value.
Owing to a low price of a personal computer and a development of multimedia goods in recent years, from the conventional personal computer using an RGB monitor as a display medium, there is increasing a request for functions such that a television monitor of a large screen and a high widespread ratio is used as a display medium and video images or the like of a TV or a VTR are fetched and superimposed onto the picture plane of the personal computer and the resultant image is displayed. However, when the display function of the television monitor is realized by an analog circuit, a circuit construction is complicated and such a complication is opposite to the realization of low costs of a system, so that it is necessary to digitize a circuit so as to form an LSI.
FIG. 1 shows an example of an information processing apparatus having a conventional television display function. A video RAM (VRAM) 112 for display is provided for a processor 110. RGB data according to a predetermined format has been stored in the VRAM 112 on a frame unit basis under the control of a VRAM control circuit 114. An NTSC signal from the outside such as television tuner, video deck, or the like is inputted to an external video input circuit 118. The external video input circuit 118 separates a vertical sync signal and a horizontal sync signal from the NTSC signal, extracts a luminance signal (Y) and color difference signals (B-Y) and (R-Y), converts the extracted signals to RGB signals, converts the RGB signals into digital RGB data, and generates the resultant data to an image processing circuit 116. The image processing circuit 116 performs a proper image process to superimpose the RGB data of the NTSC signal from the outside onto the RGB data from the VRAM 112 for display or the like and outputs the processed RGB data to a luminance chroma converting circuit 120. By the digital process, the luminance chroma converting circuit 120 converts the RGB data into chroma data (C) in which the luminance signal (Y) and the color difference signals (B-Y) and (R-Y) as chrominance signals of the NTSC system were two-phase orthogonal modulated by a frequency fsc (=3.58 MHz) of a color subcarrier. Further, the vertical and horizontal sync signals generated by an NTSC sync signal generating circuit 126 are added to the luminance data (Y). A burst signal of fsc (=3.58 Mhz) is added to the chroma data. The luminance data (Y) and chroma data (C) from the luminance chroma converting circuit 120 are converted into analog signals by a D/A converting circuit 122 and are respectively outputted as an NTSC luminance signal (Y) and an NTSC chroma signal (C) to a television monitor. In this instance, a dot clock of a frequency of 4 fsc which is four times as large as the subcarrier frequency of fsc is needed for the conversion to the luminance data (Y) in the luminance chroma converting circuit 120. A dot clock of the frequency of 4 fsc of four times is also similarly needed for the conversion to the chroma data (C). It is necessary to synchronize those dot clocks with the NTSC signal from the outside. The horizontal sync signal separated by the external video input circuit 118 is supplied to a clock analog PLL circuit 128, thereby synchronizing the phase of the dot clocks which are used in the NTSC sync signal generating circuit 126 and luminance chroma converting circuit 120 with the phase of the horizontal sync signal from the outside.
However, in such a conventional information processing apparatus having the television display function, the luminance signal of the NTSC signal from the outside ordinarily has a jitter. For a fluctuation of the phase of the horizontal sync signal due to the jitter, the phase synchronization of the system clock is performed by the clock analog PLL circuit 128. In this case, as for the conversion of the luminance signal to the luminance data by using the dot clocks obtained by frequency dividing the system clocks, even when a phase jump to synchronize the phase is performed to the jitter, there is no problem. However, a continuity of the dot clocks which are used for the chroma conversion is lost by the phase jump to perform the phase synchronization for the jitter, so that there is a problem such that an abnormal color is reproduced.
In order to guarantee the continuity of the dot clocks in the chroma conversion, when the dot clocks which are not transmitted through the clock analog PLL circuit 128 are used, there is no problem in the chroma conversion. However, since the dot clocks to be used to convert the luminance signal to the luminance data don't follow the jitter of the external luminance signal, a fluctuation of a scanning line in the horizontal direction occurs, so that a problem such that a reproducing quality deteriorates occurs.